Anechoic chambers consist of shielded rooms with their interior surfaces lined with pyramidal or wedge shaped absorber. The absorber is designed to minimize the reflectivity of the wall over a wide range of radio frequencies (RF). Traditional anechoic chamber absorber consists of polymeric foam loaded with conductive carbon, which provides good near-normal incidence absorption of specular energy. The length of the absorber is related to the frequency over which it provides good absorption, and the thicker the absorber, the lower in frequency it can absorb. For example absorber that is 6 feet (1.8 m) tall can provide significant absorption down to approximately 200 MHz, while 18 inch (46 cm) tall absorber works only down to 1 GHz.
This conventional material is fundamentally limited because of diffuse scatter (tip diffraction) that increases the overall noise level in a chamber. Diffuse scatter, unlike specular reflection, occurs in many different directions for a given incidence of the RF energy. Tip diffraction is directly related to the periodicity of the pyramidal absorber, and occurs because the inherent inhomogeneity of a patterned structure with a periodicity that is large relative to the wavelength of the incident RF energy. For example, in typical 18 inch tall absorber, the pyramids are 6 inch square (i.e. each 2-foot square piece of absorber has a 4 by 4 array of pyramids) and diffraction effects can occur at frequencies above 1 GHz. Diffraction modes are also called grating modes or Floquet modes, and their angular position can be predicted with a diffraction equation. For a simple 1-dimensional grating the diffraction equation is, d(sin θm+sin θi)=mλ, where λ is the wavelength, λi is the incident angle, θm is the diffracted angle, d is the periodicity.
Because the possible diffraction modes are only dependent on periodicity (i.e. d), no amount of reshaping of the pyramids will move these modes. The only way to reduce the number of diffractive modes is to reduce the periodicity. This is why some anechoic chamber use wedge shaped absorber, rather than pyramids to minimize diffraction. The disadvantage of wedge absorber is that the 1-dimensional geometrical taper is not as effective as the 2-dimensional taper of the pyramids, so it is less effective in reducing specular reflectivity. Design of absorber in an anechoic chamber then becomes a compromise between specular performance and diffraction effects.